It seems to me that the calculator that I have been using for my recipes (Beer Calculus) allows me to change my batch size, but does not let me adjust WHEN I add my fermentables, as to allow for higher hop utilization on a partial boil by adding a fair amount of DME and sugars at the 15/10 minute mark.

Does anyone have any input on what I should do with this? Should I just calculate this by hand? I suppose I could tell the calculator I am doing a full boil, half the water and fermentables, and use the same amount of hops since the gravity should be identical, and then add the other half of fermentables towards the end, finishing with pre-boiled water in the fermenter to achieve my batch size. Is this an appropriate way to go about this?

In this case the hop amount is in ounces and boil amount in gallons. For the correctional gravity, if your boil gravity is greater than 1.05, use the following:
corrective_gravity = 1 + ((boil_gravity - 1.05)/2)
otherwise,
correction gravity = 1

Playing with the recipe builder on BrewersHub.com (http://brewershub.com/recipe_builders/new), (which uses the above formula), if you just add dry malt extract and cascade hops, you can play with the boil gravity and see the IBU change. For Cascade hops, boiled for 60 minutes, there is a negligible change with 7 pounds of extract, and a small change (2 IBUs) boiling 15 pounds of DME.

If anyone else has a better formula, I'd be really interested to know.

I was under the impression that the massive IBU changes that happen in the calculator when I change boil size are purely due to the increased gravity, and it's pretty significant. For example, in my recipe, going from a 4.5 avg gallon boil to a 2.5 avg gallon boil reduces IBU from 22.4 to 11.1. That doesn't seem negligible to me, unless the calculator is wrong.
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CarsonMar 8 '11 at 19:57

Beer Smith v 1.3 has the same issue. I can change boil size which dramatically changes Ibu's as in Carsons example. I still have the same issue because you can only change the extract to being added before or after the boil, no where in between. Good question, I hope someone has a good workable example. Does anyone know if the new version of Beersmith is any different?
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Bullet86Mar 8 '11 at 21:14

I don't think that's an issue. You're putting the same amount of hops in a smaller volume of wort. You have a higher ratio of hops to gravity units, thus a hoppier wort.
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bakaMar 8 '11 at 21:20

1

It has always been my understanding that utilization is heavily influenced by boil volume, and much less so by gravity. Utilization improves (with diminishing returns of course) as volume increases. Pro brewers use less hops by ratio than we do to achieve the same relative IBU concentrations in production-scale batches.
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Carson C.Mar 9 '11 at 0:00

I believe you are looking for a work around not a discussion about gravity, volume and utilization rates. So here goes:

The way I work around this is to formulate my recipe as if I wasn't going to add the extract late. That way I know how much total extract I plan to use. Then I delete 80% of the extract (or whatever) and see what happens to the IBUs, and adjust the amount of hops if I feel its necessary.

Then for the brew sheet, I'll add the extract back into the recipe, but I put a comment in the notes section the amount of extract I will be adding up front then as a late addition. And I note what the IBUs will are calculated to be after doing it that way. There is no good way to do it in the current software that is available. At least that I have seen.

I'm not sure that I'm contributing much here, since both other answers were so thorough, but I wanted to point out a few things.

Most importantly, whatever route you decide to take, be consistent in your methodology from batch to batch. There are quite a few formulas that have been published for calculating IBUs, and none are 100% accurate, especially given all of the variables. Even your kettle geometry will affect utilization. You should find a method you like, and stick with it, so that even with error, you can compare relative numbers from batch to batch.

If you want to do hand calcs, I'm providing the formula that I use. I can't remember the source, but I know at the time it appeared to be the most accurate method:

where
BIT = Bitterness in IBUs
BG = gravity at the time of each addition, e.g. 1.050
TIME = boil time, in minutes
QTY = amount of hops, in oz
AA = alpha Acids of the hops, expressed as a percent
VOL = volume of wort at addition time, in gallons

Then sum BIT for each addition. You can drop the formula right into excel and replace the variables as needed.